Imaging systems capable of producing multiple colors often rely upon barrier interlayers to separate adjacent imaging chemistries. In some cases, such as those in which a dye image is formed and diffused to a receiving layer, the interlayer must not only separate imaging chemistries, but be permeable to dyes as well. Dry silver based imageable articles are one type of imaging system which may employ interlayers in this manner.
Dry silver compositions or emulsions are photothermographic compositions and contain a light-insensitive, reducible silver source; a light-sensitive silver source; and a reducing agent for the light-insensitive, reducible silver source. The light-sensitive material is generally photographic silver halide, which must be in catalytic proximity to the light-insensitive, reducible silver source. Catalytic proximity requires an intimate physical association of these two materials so that when silver specks or nuclei are generated by the irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the reducible silver source by the reducing agent. It has been long understood that silver halide is a catalyst for the reduction of silver ions and the silver-generating, light-sensitive silver halide catalyst progenitor may be placed into catalytic proximity with the light-insensitive, reducible silver source in a number of different fashions, such as by partial metathesis of the light-insensitive reducible silver source with a halogen-containing source; co-precipitation of silver halide and the light-insensitive, reducible silver source material; and other methods that intimately associate the silver halide and the light-insensitive, reducible silver source.
In both photographic and photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms. The image-wise distribution of these clusters is known in the art as a latent image. Since this latent image generally is not visible by ordinary means, the light-sensitive article must be further processed in order to produce a visual image. The visual image is produced by the catalytic reduction of silver ions which are in catalytic proximity to the silver halide grains bearing the latent image.
In color dry silver imaging systems, a leuco dye is typically incorporated as a reducing agent for the light-insensitive, reducible silver source, generally in combination with a spectral sensitizer for the silver halide. The leuco dye is oxidized upon development, thereby giving a colored image. In full color constructions, spectrally sensitized emulsion layers are typically coated onto a substrate and separated by one or more barrier interlayers.
Residual silver stain is a major problem with dry silver color constructions known in the art. This has been overcome by causing the developed dye image to diffuse from the dry silver layer to an image-receiving layer that is then stripped from the emulsion layer(s). In this case, a barrier interlayer must serve the dual roles of separating the chemistry of neighboring emulsion layers while allowing diffusion of the dye image under thermal processing conditions. In addition to the dye image, other components of the emulsion layers also diffuse to the image-receiving layer under thermal processing conditions. In this regard, unoxidized leuco dyes from the emulsion layers tend to diffuse to the image-receiving layer where they oxidize over time leading to poor print stability and color separation. Therefore, there is a need for interlayers which allow dye images, but not neutral leuco dyes, to diffuse to the image-receiving layer. Depending on the particular ingredients of a given dry silver layer, the development may be best carried out, for example, under acidic or basic conditions.
When multiple dry silver layers with incompatible developing chemistries are employed, it is very difficult to keep development conditions within the dry silver layer from affecting the development of nearby or adjacent dry silver layers. As a result, it is advantageous to coat dry silver layers with different developing conditions on opposite sides of a transparent substrate.
U.S. Pat. No. 4,594,307 discloses a heat developable photographic material that produces a pure and stable dye image by the oxidation reduction reaction between a reducible, light-insensitive organic silver salt and a leuco dye reducing agent wherein the dye formed is transferred to an image-receiving layer by continuing the heating for development in order to separate the dye formed from the silver images and other residual chemicals.
The generation of color dry silver images has been accomplished using microencapsulated constructions and tri-pack (yellow/magenta/cyan) multilayer constructions, such as those disclosed in U.S. Pat. Nos. 4,883,747 and 4,923,792. These patents disclose the use of Gantrez.TM. S-97 polystyrene; Vinol.TM. 523 partially hydrolyzed polyvinyl acetate; and Butvar.TM. B-76 polyvinyl butyral as barrier interlayers. These constructions generally have substantial silver and sensitizer stain present that affects the image color separation. The stain problem can be overcome by causing the developed dye image to diffuse from the image-forming layers into a receptor layer that is subsequently stripped from the rest of the construction. The success of this type of approach depends in large part on the barrier interlayers between the image-forming layers of the tri-pack construction selectively permitting migration of the image-forming dyes while controlling the migration of other image-forming layer components, particularly neutral leuco dyes.
U.S. Pat. Nos. 4,021,240; 4,460,681; and 5,077,178 disclose the use of multiple image-forming layers separated by barrier or interlayers.
U.S. Pat. No. 4,452,883 discloses photothermographic elements with barrier interlayers comprised of polyvinyl pyrrolidone and a methyl vinyl ether/maleic anhydride copolymer. It was taught that the maleic anhydride in the copolymer was gradually hydrolyzed over time to maleic acid, which then crosslinked with the polyvinyl pyrrolidone.
U.S. Pat. No. 4,374,921 discloses color photothermographic articles in which the various color-forming layers are separated by organic solvent soluble barrier layers that are insoluble in the organic solvent used to coat at least one of the adjacent color-forming layers. The barrier polymer disclosed therein is a copolymer of the ethyl monoester of poly(methyl vinyl ether/maleic acid) (Gantrez ES-225.TM., GAF Corp.). GAF Technical Bulletin 9642-070 (1967) at page 12 discloses that poly(N-vinylpyrrolidone) (i.e., PAP) crosslinks with polyacids such as polyacrylic acid or with Gantrez ES-225.TM. to form complexes that are insoluble in water but dissolve in dilute alkali.
U.S. Pat. No. 4,704,344 discloses the use of hydrophilic poly(N-vinyl pyrrolidone) blends and copolymers as dye receiving layers for dry silver dye diffusion systems.